Usually it means a game using 2D techniques that simulates 3D in some manner. For example, 2D games with isometric perspectives that allow height in addition to x and y, the old school DOS X-Com games are a good example. Sometimes it means something rendered in 3D that only has a 2D playing field, or a 3D world where are the actors are wholly or mostly billboarded sprites, but those usages seems less common.

I think saying 2.5D has become a way to describe what a game feels like without actually taking the time to say how it works.

Ultimately, in graphics, our games end up being shown 2D. How they get there is, even today, perspective or ortographic projections.

Billboards and 3D meshes. The bar for how much it takes on the graphics side to be qualified as 3D is probably changing, although I still regard Wolfenstein 3D as being a 3D game. It has what appears as perspective, and in a way, that's enough to describe 3D graphics it for me.

Its gameplay mechanisms are pure 2D, though. Simcity 2000 and up are regarded "2.5D" by some. Sure, a change in terrain height can mean higher land values, but you could emulate that in 2D as well. I like to work in integers when describing D. I cannot imagine half a time axis, or half a color or geometry axis. It's just plain silly, or maybe I'm not bright enough. I'd say simcity could just as well be shown/understood as a 2D game with 2D visuals.

Everything can be spoken of in as many D as you like, only after a certain count it stops making sense to me. I have this tile which travels in time. Now, it's condition dimension is 100%. It's ownership dimension is 1, for player 1. It's temerature dimensions is 20 deg celcius. Oh, and it has X and Y coordinates too, but I'm somehow still able to describe it as a line on graph, with alternating thickness, transparency and rgb values.

If it makes sense to you, go ahead and use that half dimension, -but please try to explain where I can find this dimension, and how it's actually represented in the game.

I'd still say that if it makes sense to have an altitude for objects, the game would be 3D from a geometric point of view, not 2D, even though the isometric projection results in a 2D image. All the projections I work with in GD do.

The guys on the wikipedia article seem to be all over the place. Where I'd say 2.5D doesn't mean anything really, these people apparently have a lot of opinions on the subject, changing from describing projection types and gameplay, to talking about parallax scrolling, 2D gloss effects and bump mapping. It's probably safe to say that they're not completely sure either, although overall it seems to have something to do with pseudo 3D, and making things appear to have more depth when shown in the plane.

I've never come over a paper that describes an improvement to creating lifelike geometry as "improving 2.5D graphics" or "for applications in a 2.5D environment"

Its gameplay mechanisms are pure 2D, though. Simcity 2000 and up are regarded 2.5D by some. Sure, a change in terrain height can mean higher land values, but you could emulate that in 2D as well.

I hold to my definition that if the 3rd dimension affects gameplay, it is counted as half a dimension if the rendering technique used is not full polygonal 3D.

Though it definitely is connected to the visualisation you choose to use.

Sim City 2k graphics is 2D, and the "emulation" of depth is achieved by isometric techniques.

If you would add terrain height change to any 2D game, you would need some way to show that the terrain height change had happened.

If you do this with some graphical effect, like some shading, a scaling, fog, etc, that give some illusion of depth, the game would then become a 2.5D game.

If you just have a number "5m above ocean" on every tile, then it remains a 2D game.

Yes, it seems that to many, a game being 2D, "2.5D" or 3D is directly connected to how it's rendered.

Personally, I wouldn't say using or not using polygons should have any impact to whether a game can be considered 3D or not.

A heightmap is a good example of something 2D which just as easily represents something 3D. The third dimension being the intensity of each pixel.

Had I been comfortable with viewing a heightmap myself, while playing an otherwise 2D rendered game, its mechanisms could be just as 3D as Simcity 2000.

Changing the terrain would be nothing more than altering these values, and if my brain decides to view and understand the heightmap as being 3D, then I percieve the game as 3D. (I may use bump-mapping or similar, but it won't be a fancy projection)

It always ends up as 2D, and it's up to us to have the users perceive depth, if we want them to.

I'd say that the text "5m above ocean" displayed on a tile is just as clear an indication of 3D as a heightmapped terrain.

I completely agree that you can, and should, fool the user into seeing 3D, even with the effects you mentioned, but I think the idea of 2.5D is very vague.

I think most of agree what the term 2.5D in a graphical sort of way. Ina graphical way it looks like it is 3D but is using 2D techniques.

Though if you want to get technical then we could going on forever. What defines a "dimension." Does it have to be "graphical" or "visual?" If you brain can "define" it as a dimension then it counts as one. If your brain is telling you that their is depth a long with a width and height then I would say it is technically 3D as your brain is seeing "3 dimensions." While sure it technically still may be 2D in the end it's whatever your brain thinks.

I think it is being thought of too much and that 2.5D is not meant to be a technical or real life term used to describe our physical motion. If we did that then every game we played would be 2D I guess you could say. Even if you had 3D glasses, in the end it was just "fake."

Don't think we should get too technical on this as its just pixels on a screen really...

I didn't know that it had two definitions but I guess the wiki is right.

2.5D ("two-and-a-half-dimensional"), 3/4 perspective and pseudo-3D are terms, mainly in the video game industry, used to describe either:

2D graphical projections and similar techniques are used to cause a series of images (or scenes) to simulate the appearance of being three-dimensional (3D) when in fact they are not, or gameplay in an otherwise three-dimensional video game that is restricted to a two-dimensional plane.

Trine and Trine 2 are great examples of 2.5d games. They are also very beautiful games. I think Trine 2 has the most beautiful artwork ever in a game.

Difference between 2.5 and 3d is that in 2.5 there is a side of a 3d model that is never view able due to control of the camera angles. Since this side is never viewable, it doesn't need to exist, so they can display a 3d model with some of it missing and you never can tell, this is a big optimization.

With the resources saved they can either add more detail to the view able sides, or not and keep the hardware requirements for the end user lower.

Edited by EddieV223, 20 March 2013 - 01:18 PM.

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Doom was definitely 2.5D, yeah there was a floor height (and a ceiling height) but all the collision was done in 2D (you could even punch demons from the top of a cliff that were at the bottom of the cliff! All the guns and rockets just did 2d collision, the rockets and fireballs when travelling were drawn at interpolated heights between you and the target though), you couldn't have a tunnel where you could go underneath a bridge (since there was only one floor height per collision polygon), although you could sort of do that by swapping areas of the map for other ones when you went round a corner (they did that in the N64 version at least once IIRC).

That's why it was fast on ancient hardware.

"Most people think, great God will come from the sky, take away everything, and make everybody feel high" - Bob Marley

I didn't know that it had two definitions but I guess the wiki is right.

2.5D ("two-and-a-half-dimensional"), 3/4 perspective and pseudo-3D are terms, mainly in the video game industry, used to describe either:

2D graphical projections and similar techniques are used to cause a series of images (or scenes) to simulate the appearance of being three-dimensional (3D) when in fact they are not, or gameplay in an otherwise three-dimensional video game that is restricted to a two-dimensional plane.

Until today, I was only aware of the latter. Live and learn.

The second definition is coming more and more into use for describing current games. Because 3D is still preferred for artistic reasons, but the design of the game applies a limited degree of movement as in 2D, there have been more games showing up that want to offer a "classic" gameplay experience but with modernized graphics.